1. Description of the Related Art
The present invention relates generally to travelling-wave tubes and more particularly to travelling-wave tube collectors.
2. Description of the Related Art
An exemplary traveling-wave tube (TWT) 20 is illustrated in FIG. 1. The elements of the TWT 20 are generally coaxially-arranged along a TVVT axis 21. They include an electron gun 22, a slow-wave structure 24 (embodiments of which are shown in FIGS. 2A and 2B), a beam-focusing structure 26 which surrounds the slow-wave structure 24, a signal input port 28 and a signal output port 30 which are coupled to opposite ends of the slow-wave structure 24 and a collector 32. A housing 34 is typically provided to protect the TWT elements.
In operation, a beam of electrons is launched from the electron gun 22 into the slow-wave structure 24 and is guided through that structure by the beam-focusing structure 26. A microwave input signal 36 is inserted at the input port 28 and moves along the slow-wave structure to the signal output port 30. The slow-wave structure 24 causes the phase velocity (i.e., the axial velocity of the signal's phase front) of the microwave signal to approximate the velocity of the electron beam.
As a result, the beam's electrons are velocity-modulated into bunches which overtake and interact with the slower microwave signal. In this process, kinetic energy is transferred from the electrons to the microwave signal; the signal is amplified and is coupled from the signal output port 30 as an amplified signal 38. After their passage through the slow-wave structure 24, the beam's electrons are collected in the collector 32.
The beam-focusing structure 26 is typically configured to develop an axial magnetic field. A first configuration includes a series of annular, coaxially arranged permanent magnets 40 which are separated by pole pieces 41. The magnets 40 are typically arranged so that adjacent magnet faces have the same magnetic polarity. This beam-focusing structure is comparatively light weight and is generally referred to as a periodic permanent magnet (PPM). In TWTs in which output power is more important than size and weight, a second beam-focusing configuration often replaces the PPM with a solenoid 42 (partially shown adjacent the input port 28) which carries a current supplied by a solenoid power supply (not shown).
As shown in FIGS. 2A and 2B, TWT slow-wave structures generally receive an electron beam 52 from the electron gun (22 in FIG. 1) into an axially-repetitive structure. A first exemplary slow-wave structure is the helix 43 shown in FIG. 2A. A second exemplary slow-wave structure is the coupled-cavity circuit 44 shown in FIG. 2B. The coupled-cavity circuit includes annular webs 46 which are axially spaced to form cavities 48. Each of the webs 46 forms a coupling hole 50 which couples a pair of adjacent cavities. The helix 43 is especially suited for broad-band applications while the coupled-cavity circuit is especially suited for high-power applications.
In another conventional TWT configuration, (not shown) an oscillator is formed by replacing the output port 30 with a microwave load. Random, thermally generated noise interacts with the electron beam on the slow-wave structure 24 to generate a microwave signal. Energy is transferred to this signal as it moves along the slow-wave structure. This oscillator signal generally travels in an opposite direction from that of the electron beam (i.e., the TWT functions as a backward-wave oscillator) so that the oscillator signal is coupled from the port 28.
TWTs are capable of amplifying and generating microwave signals over a considerable frequency range (e.g., 1-90 GHz). They can generate high output powers (e.g., &gt;10 megawatts) and achieve large signal gains (e.g., 60 dB) over broad bandwidths (e.g., &gt;10%).
The electron gun 22, the signal input port 28, the signal output port 30 and the collector 32 of FIG. 1 and the helix 43 of FIG. 2A, are again shown in the TWT schematic 20 of FIG. 3 (for clarity of illustration, the slow-wave structure is not shown in the schematic). As described above with reference to FIGS. 1 and 2A, the helix 43 is an exemplary slow-wave structure and the signal input port 28 and signal output port 30 are coupled to opposite ends of this exemplary slow-wave structure, has a cathode 56 and an anode 58 and the collector 32 has a first annular stage 60, a second annular stage 62 and a third stage 64. Because the third stage 64 generally has a cup-like or bucket-like form, it is sometimes referred to as the "bucket" or "bucket stage".
The helix 43 and a body 70 of the TWT are at ground potential. The cathode 56 is biased negatively by a voltage V.sub.cath from a cathode power supply 74, as indicated by + and - potential indicators. An anode power supply 76 is referenced to the cathode 56 and applies a positive voltage to the anode 58. This positive voltage establishes an acceleration region 78 between the cathode 56 and the anode 58. Electrons are emitted by the cathode 56 and accelerated across the acceleration region 78 to form the electron beam 52.
The electron beam 52 travels through the helix 43 and exchanges energy with a microwave signal which travels along the helix 43 from an input port 28 to an output port 30. Only a portion of the kinetic energy of the electron beam 52 is lost in this energy exchange. Most of the kinetic energy remains in the electron beam 52 as it enters the collector 32. A significant part of this kinetic energy can be recovered by decelerating the electrons before they are collected at the collector walls.
Because of their negative charge, the electrons of the electron beam 52 form a negative "space charge" which would radially disperse the electron beam 52 in the absence of any external restraint. Accordingly, the beam-focusing structure applies an axially-directed magnetic field which restrains the radial divergence of electrons by causing them to spiral about the beam.
However, the electron beam 52 is no longer under this restraint when it enters the collector 32 and, consequently, it begins to radially disperse. In addition, the interaction between the electron beam 52 and the microwave signal on the slow-wave structure 24 causes the beam's electrons to have a "velocity spread" as they enter the collector 32, i.e., the electrons have a range of velocities and kinetic energies.
Electron deceleration is achieved by application of negative voltages to the collector. The potential of the collector is "depressed" from that of the TWT body 70 (i.e., made negative relative to the body 70). The kinetic energy recovery is further enhanced by using a multistage collector, e.g., the collector 32, in which each successive stage is further depressed from the body potential of V.sub.B. For example, if the first collector stage 60 has a potential V.sub.1, the second collector stage 62 a potential V.sub.2 and the third collector stage 64 a potential of V.sub.3, these potentials are typically related by the equation V.sub.B =0&gt;V.sub.1 &gt;V.sub.2 &gt;V.sub.3 as indicated in FIG. 3.
The voltage V.sub.1 on the first stage 60 is depressed sufficiently to decelerate the slowest electrons 80 in the electron beam 52 and yet still collect them. If this voltage V.sub.1 is depressed too far, the electrons 80 will be repelled from the first stage 60 rather than being collected by it. These repelled electrons may flow to the body 70 and this will reduce the TWT's efficiency. Alternatively, they may reenter the energy exchange area of the helix 43. This undesirable feedback will reduce the TWT's stability.
Similar to the first stage 60, successively depressed voltages are applied to successive collector stages to decelerate (but still collect) successively faster electrons in the electron beam 52, e.g., electrons 82 are collected by collector stage 62 and electrons 84 are collected by collector stage 64.
In operation, the diverging low kinetic energy electrons 80 are repelled by collector stage 62, which causes their divergent path to be modified so that they are collected on the interior face of the less depressed collector stage 60. Higher energy electrons 82 are repelled by collector stage 64, which causes their divergent paths to be modified so that they are collected on the interior face of the less depressed collector stage 62. Finally, the highest energy electrons 84 are decelerated and collected by the collector stage 64. This process of improving TWT efficiency by decelerating and collecting successively faster electrons with successively greater depression on successive collector stages is generally referred to as "velocity sorting".
The efficiency gain realized by velocity sorting of the electron beam 52 can be further understood with reference to current flows through the collector power supply 88 which is coupled as indicated by + and - potential indicators, between the cathode 56 and the collector stages 60, 62 and 64. If the potential of the collector 32 were the same as the collector body 70, the total collector electron current I.sub.coll would flow back to the cathode power supply 74 as indicated by the current 90 in FIG. 3, and the input power to the TWT 20 would substantially be the product of the cathode voltage V.sub.cath and the collector current I.sub.coll.
In contrast, the currents of the multistage collector 32 flow through the collector power supply 88. The input power associated with each collector stage is the product of that stage's current and its associated voltage in the collector power supply 88. Because the voltages V.sub.1, V.sub.2 and V.sub.3 of the collector power supply 88 are a fraction (e.g., in the range of 30-70%) of the voltage of the cathode power supply 74, the TWT input power is effectively decreased.
Efficiencies of TWTs with multistage collectors are typically in the range of 25-60%, with higher efficiency generally associated with narrower bandwidth. These efficiencies can be further improved by enhancing the velocity sorting of the collector and considerable efforts have been expended towards this goal in the areas of collector design, simulation and prototype test.
In some collectors, velocity sorting is improved by configuring a collector stage to introduce radial asymmetries of the electric field within that stage. These radial asymmetries can often enhance velocity sorting by selectively moving electrons away from the electron beam's axis.
For example, some of the low kinetic energy electrons 80 in FIG. 3 may travel along the collector axis (generally, the axis 21 of FIG. 1). When these coaxial electrons are repelled by the higher depressed collector stages, they may reverse their path and travel back along the collector axis into the energy exchange area of the helix 43. A radial asymmetry in the electric field will cause these electrons to diverge from the collector axis and increase the probability that they will be collected by the collector stage 60.
Radial field asymmetries (electric or magnetic) are conventionally realized, for example, by beveling the leading edge of the first collector stage's aperture 92 as indicated by the broken line 93 in FIG. 3, or by attaching external magnets to the collector body. Although these structures can improve velocity sorting, the former cannot be easily modified and the latter is expensive, time consuming and adds weight and parts complexity.
Because the efficiency of a collector is a function of many elements, (e.g., diameter, length and shape of each stage, spatial interrelationship of stages, stage materials and interaction variations in the slow-wave structure), even complex computer modeling does not completely predict a design's performance. In addition, 3-dimensional computer models are typically limited to simulation of symmetric designs.
Even well-designed velocity sorting may be degraded by the introduction of unexpected a symmetries, e.g., by manufacturing tolerances. Consequently, extensive and expensive prototype testing and design modification are often required to finalize a collector design and time-consuming test adjustments (e.g., attachment of external magnets) are often required during production because of the lack of any ready means for adjusting a collector's radial electric field distributions.